This invention relates to a method of permanently characterizing or lettering workpieces which are subjected to a heat treatment in the course of their processing. More specifically, this invention relates to an automobile glass pane characterized by the method of the invention.
The characterization of workpieces before they are processed in various process stations may be advisable for various reasons. For example, it may be desirable to provide workpieces with individual characterization and/or information data for the purpose of following the progress of the product during its production operation. Such a following of the progress of the product is desirable for quality assurance so that if any production faults occur, the cause of the faults can be rapidly and clearly recognized.
Typically, in the prior art, organic paints or inks provide means for .characterization. Identifying numbers, letters and/or other indicia are simply painted onto a workpiece by processes known to those skilled in the art. The problem with characterizing workpieces in this manner occurs if the workpieces are subjected, during the course of their processing, to a heat treatment at a high temperature of about 500.degree. to 600.degree. C. For example, automobile glass panes are heat processed to provide bent and/or toughened glass panes. Also, workpieces of metal are heated to a high temperature during machining. In these cases, the usual methods of characterization or identification by means of organic paints or inks fail because these paints or inks burn at heat treatment temperatures, and the characterization is no longer visible.
A permanent characterization or identification is possible, even at these high temperatures, by printing with an inorganic bake-on paint which melts and bakes in at heat treatment temperatures. However, the bake-on paints commonly used for this purpose can only be applied with the, help of specific printing methods, specifically by means of a screen printing method known in the art. Unfortunately, the screen printing method is unsuitable for the purposes of individually labelling workpieces (e.g., in the example used above where unique characterization is utilized to follow the progress of a product during production). Here, a continually changing characterization or identification must be carried out so as to identify each individual workpiece. In this case, a different printing screen would be required for every workpiece, which is unacceptable because of the high cost of screen printing.